Sliding member

ABSTRACT

A sliding member includes a back-metal layer and a sliding layer made of a copper alloy. The back-metal layer is made of a hypoeutectoid steel including 0.07 to 0.35 mass % of carbon, and has a structure including a ferrite phase and pearlite. The back-metal layer includes a pore existing region including a plurality of closed pores that are not open to a bonding surface when viewing a cross-section perpendicular to a sliding surface. The closed pores have an average size of 5 to 15 μm. The pore existing region extends from the bonding surface toward an inner portion of the back-metal layer, and has a thickness of 10 to 60 μm. A ratio V 2 /V 1  of a total volume V 2  of the closed pores to a volume V 1  of the pore existing region is 0.05 to 0.1.

TECHNICAL FIELD

The present invention relates to a sliding member for a bearing, forexample, used in internal combustion engines or automatic transmissions,or a bearing used in various machines. Specifically, the presentinvention relates to a sliding member including a sliding layer on aback-metal layer.

BACKGROUND ART

A sliding member such as sliding bearing has been used for a bearingdevice of an internal combustion engine, an automatic transmission, orthe like. Such sliding bearing is formed in a cylindrical orsemi-cylindrical shape from a sliding material including a copper alloysliding layer on a steel back-metal layer. For example, JP 6-322462A andJP 2002-220631A describe a sliding member including a sliding layer madeof a copper-lead bearing alloy or phosphor bronze. In such a slidingmember, the sliding layer made of a copper alloy achieves seizureresistance and wear resistance as well as sliding properties, while theback-metal layer functions as a support of the copper alloy and impartsstrength to the sliding member.

During operation of the internal combustion engine or the automatictransmission, the sliding member bears a dynamic load from a countershaft member on the sliding surface of the sliding layer. For example, asliding bearing is mounted to a cylindrical bearing holding hole of abearing housing of the internal combustion engine or the automatictransmission, and bears a dynamic load from a rotating counter shaftmember. In recent years, the internal combustion engine and theautomatic transmission have a smaller weight in order to reduce fuelconsumption, leading to lower rigidity of the bearing housing than thatof a conventional bearing housing. Thus, in the bearing device of theinternal combustion engine and the automatic transmission connected tothe internal combustion engine, the bearing housing is more likely to beelastically deformed by a dynamic load from the counter shaft memberduring operation of the internal combustion engine. The sliding member(sliding bearing) mounted to the bearing holding hole of the bearinghousing is elastically deformed in a circumferential direction due tothe deformation of the bearing housing. In a conventional slidingmember, when a varying circumferential force is applied to the slidingbearing, a difference in the amount of elastic deformation between thesliding layer made of the copper alloy and the steel back-metal layer,in some cases, leads to shear failure at an interface between thesliding layer and the back-metal layer, resulting in breakage of thesliding member.

JP 2006-22896A has an object of improving bonding strength between abearing alloy layer and a steel back-metal layer. In JP 2006-22896A, aCu—Sn—Fe-based alloy is used as a copper alloy, and a Sn—Fe compound isprecipitated by heat treatment to cause the copper alloy to have finergrains, thereby improving bonding strength between the bearing alloylayer and the steel back-metal layer.

BRIEF SUMMARY OF THE INVENTION

The method described in JP 2006-22896A can improve bonding strengthbetween the bearing alloy layer and the steel back-metal layer. However,the method is insufficient to prevent the occurrence of shear failurebetween the bearing alloy layer and the steel back-metal layer when adynamic load is applied. Therefore, an object of the present inventionis to provide a sliding member having more improved bonding between asliding layer and a back-metal layer than in a conventional slidingmember.

According to an aspect of the present invention, provided is a slidingmember including: a back-metal layer having a back surface and a bondingsurface; and a sliding layer that is made of a copper alloy and isprovided on the bonding surface of the back-metal layer. The slidinglayer has a sliding surface. The copper alloy includes 0.5 to 12 mass %of Sn, 0.5 to 15 mass % of Ni, 0.06 to 0.2 mass % of P, and the balanceof Cu and inevitable impurities. The back-metal layer is made of ahypoeutectoid steel including 0.07 to 0.35 mass % of carbon, and has astructure including a ferrite phase and pearlite. The back-metal layerhas a pore existing region that extends from the bonding surface towardan inner portion of the back-metal layer. In cross-sectional viewperpendicular to the sliding surface, the pore existing region has aplurality of closed pores that are not open to the bonding surface andhave an average size of 5 to 15 μm, and the pore existing region has athickness of 10 to 60 μm. A ratio (V2/V1) of a total volume V2 of theclosed pores to a volume V1 of the pore existing region is 0.05 to 0.1.

The sliding member according to the present invention has the poreexisting region on the bonding surface of the back-metal layer servingas an interface with the sliding layer. Due to the plurality of closedpores of the pore existing region, the pore existing region is morelikely to be elastically deformed, and a difference in elasticdeformation is small between the copper alloy of the sliding layer andthe pore existing region. Thus, when an external force is applied to thesliding member, a difference in elastic deformation is small between thecopper alloy of the sliding layer and the pore existing region of theback-metal layer, and shear failure is less likely to occur between thecopper alloy of the sliding layer and the back-metal layer.

According to an embodiment of the present invention, an average distancebetween the closed pores and the bonding surface is preferably 5 to 15μm in cross-sectional view perpendicular to the sliding surface.

According to an embodiment of the present invention, an average distancebetween adjacent ones of the closed pores in a direction parallel to thebonding surface is preferably 5 to 15 μm in cross-sectional viewperpendicular to the sliding surface.

According to an embodiment of the present invention, an average aspectratio of the closed pores is preferably not more than 2.5 incross-sectional view perpendicular to the sliding surface.

According to an embodiment of the present invention, a volume ratio Pcof pearlite in a structure of a center portion in a thickness directionof the back-metal layer and a volume ratio Ps of pearlite in the poreexisting region preferably satisfy the following relation:

Ps/Pc≤0.5.

According to an embodiment of the present invention, a region of theback-metal layer excluding the pore existing region preferably includes0.07 to 0.35 mass % of C, not more than 0.4 mass % of Si, not more than1 mass % of Mn, not more than 0.04 mass % of P, not more than 0.05 mass% of S, and the balance of Fe and inevitable impurities.

According to an embodiment of the present invention, the copper alloypreferably further includes one or more selected from 0.01 to 10 mass %of Fe, 0.01 to 5 mass % of Al, 0.01 to 5 mass % of Si, 0.1 to 5 mass %of Mn, 0.1 to 30 mass % of Zn, 0.1 to 5 mass % of Sb, 0.1 to 5 mass % ofIn, 0.1 to 5 mass % of Ag, 0.5 to 25 mass % of Pb, and 0.5 to 20 mass %of Bi.

Other objects, features and advantages of the present invention willbecome apparent from the following description of non-limitingembodiments of the present invention with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a cross section perpendicular to asliding surface of an example of a sliding member according to thepresent invention.

FIG. 2 is a schematic diagram of a cross-sectional structure in thevicinity of a pore existing region of a back-metal layer shown in FIG.1.

FIG. 2A is a schematic diagram of a cross-sectional structure of acenter portion in a thickness direction of the back-metal layer shown inFIG. 1.

FIG. 3 is a diagram showing a distance L1 between a closed pore and abonding surface, and a distance L2 between closed pores.

FIG. 4 is a diagram showing an aspect ratio Al of the closed pore.

FIG. 5 is a schematic diagram of a cross-sectional structure in thevicinity of a pore existing region of a back-metal layer of anotherexample of the sliding member according to the present invention

FIG. 6 is a schematic diagram of a cross section perpendicular to asliding surface of a conventional sliding member.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 6 is a schematic diagram of a cross section of a conventionalsliding member 11. The sliding member 11 includes a sliding layer 13including a copper alloy 14 on one surface of a back-metal layer 12. Theback-metal layer 12 includes a hypoeutectoid steel including 0.07 to0.35 mass % of carbon, and has a structure of a typical hypoeutectoidsteel. That is, the structure of the back-metal layer 12 includes mainlya ferrite phase 6 and granular pearlite 7 is dispersed in a matrix ofthe ferrite phase (see FIG. 2A). The structure is uniformly formedthroughout a thickness direction of the back-metal layer 12. Thus, theback-metal layer 12 has approximately uniform deformation resistanceagainst an external force throughout the thickness direction of theback-metal layer 12.

As described above, a bearing housing is more likely to be elasticallydeformed by a dynamic load from a counter shaft member during operationof a bearing device. Thus, in the case of the conventional slidingmember 11, a varying circumferential force is applied, due todeformation of a bearing housing, to the sliding member (slidingbearing) mounted to a bearing holding hole of the bearing housing,leading to elastic deformation of the sliding member. In theconventional sliding member 11, the back-metal layer 12 has a structureof a typical hypoeutectoid steel, and has high strength and highdeformation resistance as compared with the copper alloy 14 of thesliding layer 13. Thus, a difference in the elastic deformation is largebetween the back-metal layer 12 and the copper alloy 14 of the slidinglayer 13 at an interface between the back-metal layer 12 and the slidinglayer 13. Therefore, shear failure is more likely to occur between theback-metal layer 12 and the sliding layer 13.

An embodiment of a sliding member 1 according to the present inventionis described below with reference to FIGS. 1, 2, and 2A. FIG. 1 is aschematic diagram showing a cross section of the sliding member 1including a sliding layer 3 made of a copper alloy 4 on a back-metallayer 2. The sliding layer 3 has a sliding surface 31 on a side oppositeto the back-metal layer 2. The back-metal layer 2 has a surface (bondingsurface 21) on which the sliding layer 3 is formed, and a back surface22 on a side opposite to the bonding surface 21. On the bonding surface21 of the back-metal layer 2 which serves as an interface between theback-metal layer 2 and the copper alloy layer 4, a pore existing region5 (described below) is formed.

The sliding member of the present invention may optionally include, onthe surface of the sliding layer and/or the surface of back-metal layer,a coating layer of Sn, Bi or Pb or an alloys based on these metals, or acoating layer of a synthetic resin or a synthetic resin based coating.In this case, the surface of the sliding layer 3 is herein referred toas “sliding surface 31”.

FIG. 2 is an enlarged view showing a structure in the vicinity of thebonding surface 21 of the back-metal layer 2. The back-metal layer 2 hasa pore existing region that extends over a certain distance from thebonding surface 21 toward an inner portion of the back-metal layer 2 andhas a large number of closed pores. On the other hand, FIG. 2A is anenlarged view showing a structure of a center portion in a thicknessdirection of the back-metal layer 2 (hereinafter simply referred to as“center portion of the back-metal layer 2”). In FIGS. 2 and 2A, theferrite phase 6 and the pearlite 7 in the structure are exaggerated foreasier understanding.

The copper alloy 4 of the sliding layer 3 has a composition including0.5 to 12 mass % of Sn, 0.5 to 15 mass % of Ni, 0.06 to 0.2 mass % of P,and the balance of Cu and inevitable impurities. Sn, Ni, and P includedin the copper alloy are elements that increase strength of the copperalloy. If the amount of these elements is less than the lower limit, theeffect of the elements is insufficient, and if the amount of theelements is more than the upper limit, the copper alloy is brittle.During sintering (described later), Ni and P in the copper alloy 4 ofthe sliding layer 3 diffuse in the vicinity of the bonding surface ofthe back-metal layer 2 serving as the interface with the copper alloylayer 4. It relates to formation of the pore existing region 5 of theback-metal layer 2 (described later).

In some cases, elements included in the back-metal layer 2 diffuse in aportion of the copper alloy 4 of the sliding layer 3 in the vicinity ofthe bonding surface of the back-metal layer 2 during sintering. Such acase is also included in the scope of the present invention.

The copper alloy 4 may have a composition including, for example, 0.5 to12 mass % of Sn, 0.5 to 15 mass % of Ni, and 0.06 to 0.2 mass % of P,and optionally one or more elements selected from 0.01 to 10 mass % ofFe, 0.01 to 5 mass % of Al, 0.01 to 5 mass % of Si, 0.1 to 5 mass % ofMn, 0.1 to 30 mass % of Zn, 0.1 to 5 mass % of Sb, 0.1 to 5 mass % ofIn, 0.1 to 5 mass % of Ag, 0.5 to 25 mass % of Pb, and 0.5 to 20 mass %of Bi. Fe, Al, Si, Mn, Zn, Sb, In and Ag increase strength of the copperalloy 4. If the amount of these elements is less than the lower limit,the effect of these elements is insufficient, and if the amount of theseelements is more than the upper limit, the copper alloy 4 is brittle. Pband Bi increase lubricating properties of the copper alloy 4. If theamount of Pb and Bi is less than the lower limit, the effect of Pb andBi is insufficient, and if the amount of Pb and Bi is more than theupper limit, the copper alloy 4 is brittle. When the copper alloy 4includes two or more of these selected elements, a total amount of theelements is preferably not more than 40 mass %.

The sliding layer 3 may optionally further include 0.1 to 10 volume % ofone or more types of hard particles selected from Al₂O₃, SiO₂, AlN,Mo₂C, WC, Fe₂P, and Fe₃P. The hard particles are dispersed in a matrixof the copper alloy 4 of the sliding layer 3 and increase wearresistance of the sliding layer 3. If the amount of hard particlesincluded in the copper alloy 4 is less than the lower limit, the effectof the hard particles is insufficient, and if the amount of hardparticles included in the copper alloy 4 is more than the upper limit,the sliding layer 3 is brittle.

The sliding layer 3 may optionally further include 0.1 to 10 volume % ofone or more solid lubricants selected from MoS₂, WS₂, graphite, andh-BN. The solid lubricant is dispersed in the matrix of the copper alloy4 of the sliding layer 3 and increases lubricating properties of thesliding layer 3. If the amount of solid lubricant included in the copperalloy 4 is less than the lower limit, the effect of the solid lubricantis insufficient, and if the amount of solid lubricant included in thecopper alloy 4 is more than the upper limit, the sliding layer 3 isbrittle.

The back-metal layer 2 includes a hypoeutectoid steel including 0.07 to0.35 mass % of carbon. As shown in FIG. 2A, the back-metal layer 2 has astructure including the ferrite phase 6 and the pearlite 7. If theback-metal layer 2 includes a hypoeutectoid steel including less than0.07 mass % of carbon, the back-metal layer 2 has low strength, leadingto insufficient strength of the sliding member 1. On the other hand, ifthe back-metal layer 2 includes a hypoeutectoid steel including morethan 0.35 mass % of carbon, the back-metal layer 2 is brittle.

As described above, Ni and P in the copper alloy 4 of the sliding layer3 diffuse, during sintering, into a portion (the pore existing region 5)in the vicinity of the bonding surface of the back-metal layer 2 servingas the interface with the copper alloy layer 4. Thus, the pore existingregion 5 includes a larger amount of Ni and P than a region (hereinafterreferred to as “main region”) of the back-metal layer 2 excluding thepore existing region. The main region of the back-metal layer 2 may havea composition including 0.07 to 0.35 mass % of carbon, and one or moreelements of not more than 0.4 mass % of Si, not more than 1 mass % ofMn, not more than 0.04 mass % of P, and not more than 0.05 mass % of S,and the balance of Fe and inevitable impurities. The structure of theback-metal layer 2 includes the ferrite phase 6 and the pearlite 7.Please note that this does not exclude that the structure of theback-metal layer 2 includes fine precipitates (that cannot be detectedby observation of the structure at a magnification of 1000 times withuse of a scanning electron microscope).

The ferrite phase 6 of the back-metal layer 2 includes only 0.02 mass %of carbon at maximum, and has a composition close to that of pure iron.On the other hand, the pearlite 7 of the back-metal layer 2 has alamellar structure in which a ferrite phase and a cementite (Fe₃C)phase, which is an iron carbide, are alternately arranged to form a thinplate, and has higher strength than the ferrite phase 6.

In a cross-sectional structure shown in FIG. 2, a volume ratio betweenthe ferrite phase 6 and the pearlite 7 is approximately the same betweenin the pore existing region 5 (except for closed pores 51) and in themain region of the back-metal layer 2.

The pore existing region 5 has the plurality of closed pores 51. Theclosed pores 51 are formed in grains or between grains of the ferritephase 6 close to the bonding surface 21, and are not in contact with thebonding surface 21, that is, are not open to the bonding surface 21. Ina cross-sectional view perpendicular to the sliding surface 31 (i.e.,perpendicular to the bonding surface 21 of the back-metal layer 2), manyof the plurality of closed pores 51 are spaced approximately the samedistance from the bonding surface 21, and many of the plurality ofclosed pores 51 are spaced approximately the same distance also in adirection parallel to the bonding surface 21 (see FIG. 2).

The pore existing region 5 is a region that is defined to include theclosed pores 51 and has a constant thickness T1 from the bonding surface21 toward the inner portion of the back-metal layer 2 in cross-sectionalview perpendicular to the sliding surface 31. The thickness T1 of thepore existing region 5 is 10 to 60 μm. The thickness T1 is morepreferably 15 to 45 μm. Specifically, when an imaginary line L (dottedline L in FIG. 2) is drawn in contact with a contour of the closed pore51 farthest from the bonding surface 21 (on an inner side of theback-metal layer 2) in the cross-sectional structure and parallel to thebonding surface 21 in cross-sectional view perpendicular to the slidingsurface 31, the pore existing region 5 is a region that extends from thebonding surface 21 until the imaginary line L. The closed pores 51 arenot observed in the structure of the back-metal layer other than thepore existing region 5. In some cases, voids or small holes such asminute flaws having a size of less than 0.5 μm may be observed, but suchvoids or small holes are not considered as the closed pores 51.

In a typical sliding member, the back-metal layer 2 has a thickness of0.7 mm at minimum. When the thickness T1 of the pore existing region 5is in the range of 10 to 60 μm, the pore existing region 5 has littleinfluence on the strength of the back-metal layer 2. Furthermore, aratio X1 of the thickness T1 of the pore existing region 5 to athickness T of the back-metal layer is preferably not more than 0.07.

During sintering, Ni and P in the copper alloy 4 of the sliding layer 3diffuse to solid-solve in the ferrite phase 6 in the pore existingregion 5 of the back-metal layer 2. The pore existing region 5 (exceptfor the closed pores 51) includes approximately 0.5 to 5 mass % of Niand approximately 0.02 to 0.25 mass % of P.

Diffusion of Ni and P of the copper alloy 4 into the vicinity of thebonding surface 21 of the back-metal layer 2 can be observed bycomposition measuring a plurality of portions (e.g., 5 portions) of across-sectional structure perpendicular to the sliding surface 31 of thesliding member with use of an EPMA (electron probe microanalyzer).

In some cases, Cu, Sn, or the selected elements described above includedin the copper alloy diffuse, in addition to Ni and P, in the vicinity ofthe bonding surface 21 of the back-metal layer 2 serving as theinterface with the sliding layer 3 (i.e., in the vicinity of the poreexisting region 5) during sintering (described later), and such a caseis also included in the scope of the present invention.

A ratio of a total volume V2 of the closed pores 51 to a volume V1 ofthe pore existing region 5 is 5 to 10% (V2/V1=0.05 to 0.1). If the ratioV2/V1 is less than 5%, the pore existing region 5 has excessively highdeformation resistance, leading to an insufficient effect of achieving asmall difference in deformation resistance between the pore existingregion 5 and the copper alloy 4 of the sliding layer 3 (i.e., differencein elastic deformation when an external force is applied to the slidingmember). If the ratio V2/V1 is more than 10%, the pore existing region 5of the back-metal layer 2 has excessively low deformation resistance,and thus when an external force is applied to the sliding member, insome cases, buckling (plastic deformation) occurs in the back-metallayer 2 (ferrite phase 6) around the closed pores 51.

The closed pores 51 have an average size of 5 to 15 μm. If the averagesize of the closed pores 51 is less than 5 μm, the ratio V2/V1 is morelikely to be less than 5%, which is not preferable. If the average sizeof the closed pores 51 is more than 15 μm, the ratio V2/V1 is morelikely to be more than 10%, which is not preferable.

Apparently, a maximum diameter of the closed pores 51 is smaller thanthe thickness T1 of the pore existing region 5.

The ratio V2/V1 of the total volume V2 of the closed pores 51 to thevolume V1 of the pore existing region 5 is obtained in the followingmanner. First, a cross-sectional structure (which was cut, polished andetched with a nital solution which is a mixed solution of ethanol and 3%nitric acid) at a plurality of portions (e.g., 5 portions) cut in adirection parallel to a thickness direction of the sliding member 1(i.e., a direction perpendicular to the sliding surface 31) is prepared.Next, electronic images of a portion in the vicinity of the bondingsurface 21 of the back-metal layer 2 of the cross-sectional structureare taken at a magnification of 500 times with use of an electronmicroscope. Then, the pore existing region 5 is determined as describedabove in the images with use of a general image analysis method(analysis software: Image-Pro Plus (Version 4.5) manufactured byPlanetron, Inc.). Subsequently, a thickness of the pore existing region5 from the bonding surface 21 is measured, and then an area ratio of theclosed pores 51 in the structure of the pore existing region 5 ismeasured. A value of the area ratio corresponds to the ratio of thetotal volume V2 of the closed pores 51 to the total volume V1 of thepore existing region 5. The magnification of the electronic images isnot limited to 500 times, and may be changed to other magnifications.

In order to obtain the average size of the closed pores 51, theelectronic images of the cross section is prepared with use of the abovemethod, and an area of each of the closed pores 51 in the electronicimages is measured with use of the image analysis method, and then adiameter of a circle having an area equal to the area of each of theclosed pores 51 (equivalent circle diameter) is calculating. Asdescribed above, pores having a size of less than 0.5 μm are notconsidered as the closed pores 51.

An average distance L1 of the closed pores 51 from the bonding surface21 is preferably 2.5 to 25 μm, and more preferably 5 to 15 μm. Theaverage distance L1 from the bonding surface 21 is a distance in adirection perpendicular to the bonding surface 21 in cross-sectionalview perpendicular to the sliding surface 31 (see FIG. 3). The averagedistance L1 is obtained by using the electronic images obtained with useof the above method. A length from a portion of each of the closed pores51 (i.e., a contour of each of the closed pores) closest to the bondingsurface 21 to the bonding surface 21 is measured in the electronicimages with use of the image analysis method, and then averaging themeasured lengths.

If the average distance L1 is less than 2.5 μm, a steel material betweenthe bonding surface 21 and the closed pores 51 (i.e., a matrix portionin the pore existing region 5) of the back-metal layer 2 has anexcessively small thickness. Thus, when an external force is applied tothe sliding member, in some cases, buckling (plastic deformation) occursbetween the bonding surface 21 and the closed pores 51. If the averagedistance L1 is more than 25 μm, the steel material between the bondingsurface 21 and the closed pores 51 (i.e., the matrix portion in the poreexisting region 5) has an excessively large thickness, leading to aninsufficient effect of achieving a small difference in deformationresistance between the pore existing region 5 and the copper alloy 4 ofthe sliding layer 3 (i.e., difference in elastic deformation when anexternal force is applied to the sliding member).

An average distance L2 between adjacent closed pores 51 is preferably2.5 to 25 μm, and more preferably 5 to 15 μm. The average distance L2between adjacent closed pores 51 is defined as a distance in a directionparallel to the bonding surface 21 in cross-sectional view perpendicularto the sliding surface 31 (see FIG. 3). The average distance L2 isobtained by using the electronic images with use of the above method,and a length in a direction parallel to the bonding surface 21 between afirst closed pore 51 and a second closed pore 51 located closest to thefirst closed pore 51 (i.e., a length between contours (contour lines) ofthe closed pores 51) is measured in the electronic images with use ofthe image analysis method, and then the measured lengths are averaged.

If the average distance L2 is less than 2.5 μm, the steel materialbetween the closed pores 51 (i.e., the matrix portion in the poreexisting region 5) has an excessively small thickness (length), and thuswhen an external force is applied to the sliding member, in some cases,buckling (plastic deformation) of the back-metal layer 2 occurs betweenthe closed pores 51. The average distance L2 of more than 25 μm leads toan insufficient effect of achieving a small difference in deformationresistance between the pore existing region 5 and the copper alloy 4 ofthe sliding layer 3 (i.e., difference in elastic deformation when anexternal force is applied to the sliding member).

An average aspect ratio Al of the closed pores 51 is preferably not morethan 3, and more preferably not more than 2.5. The average aspect ratioAl of the closed pores 51 is represented by an average of the ratiosbetween a major axis and a minor axis of the closed pores 51 incross-sectional view perpendicular to the sliding surface 31. In a casewhere the closed pores 51 are substantially spherical, when an externalforce is applied to the sliding member, deformation of the closed pores51 (deformation of the matrix portion in the pore existing region 5around the closed pores 51) is less likely to occur. The closed pores 51are preferably arranged so that the major axes of many of the closedpores 51 are oriented in a direction substantially parallel to thebonding surface 21.

The aspect ratio Al of the closed pores 51 is obtained by using theelectronic images obtained with use of the above method, and an averageof the ratios between a major axis length L3 and a minor axis length S1(major axis length L3/minor axis length S1) of the closed pores 51 inthe electronic images is measured with use of the image analysis method(see FIG. 4). The major axis length L3 of the closed pore indicates alength of the closed pore at a position at which the closed pore has amaximum length in the electronic images. The minor axis length S1 of theclosed pore indicates a length of the closed pore at a position at whichthe closed pore has a maximum length in a direction orthogonal to thedirection of the major axis length L3.

FIG. 5 is a cross-sectional schematic diagram of another embodiment ofthe sliding member according to the present invention. This embodimentdiffers from the embodiment shown in FIG. 2 in that a structure in thepore existing region 5 of the back-metal layer 2 includes a lower ratioof pearlite 7. Other configurations in FIG. 5 are same as those of theembodiment in FIG. 2, and is thus not described.

FIG. 5 is an enlarged view showing a structure of the pore existingregion 5 in the vicinity of the bonding surface 21 of the back-metallayer 2. In FIG. 5, the ferrite phase 6 and the pearlite 7 in thestructure are exaggerated for easier understanding. A structure of acenter portion in the thickness direction of the back-metal layer 2(hereinafter simply referred to as “center portion of the back-metallayer 2”) is the same as the structure shown in FIG. 2A.

A volume ratio of the pearlite 7 in the structure of the pore existingregion 5 is lower by not less than 50% than a volume ratio of thepearlite 7 in the structure of the center portion of the back-metallayer 2. That is, a volume ratio Pc of the pearlite in the structure ofthe center portion of the back-metal layer 2 and a volume ratio Ps ofthe pearlite in the pore existing region 5 satisfy Ps/Pc≤0.5.

An area ratio of the pearlite 7 in the structure is obtained in thefollowing manner. First, a cross-sectional structure at a plurality ofportions (e.g., 5 portions) cut in a direction parallel to a thicknessdirection of the sliding member 1 (i.e., a direction perpendicular tothe sliding surface 31) is prepared. Next, electronic images of each ofthe center portion of the back-metal layer 2 and the pore existingregion 5 of the back-metal layer 2 of the cross-sectional structure aretaken at a magnification of 500 times with use of an electronmicroscope. Then, the area ratio of the pearlite 7 in the structure ismeasured in the images with use of a general image analysis method(analysis software: Image-Pro Plus (Version 4.5) manufactured byPlanetron, Inc.). Thus, it can be confirmed that the area ratio of thepearlite 7 in the structure in the pore existing region 5 (except forthe closed pores 51) is lower by not less than 50% than the area ratioof the pearlite 7 in the structure of the center portion of theback-metal layer 2.

The center portion of the back-metal layer 2 may not necessarily be astrictly at a center position in the thickness direction of theback-metal layer 2. This is because a structure between the back surface22 and a portion in the vicinity of the pore existing region 5 of theback-metal layer 2 is substantially the same structure (the area ratioof ferrite phase 6/pearlite 7 is approximately the same). Thus, the“center portion of the back-metal layer 2” herein includes the portionlocated at the center position in the thickness direction of theback-metal layer 2 as well as a portion in the vicinity of the centerposition. In the above observation, the volume ratio of the pearlite 7in the structure is measured as the area ratio in cross-sectional view,and the value of the area ratio corresponds to a volume ratio of thepearlite 7 in the structure.

The back-metal layer 2 has a portion (low-pearlite region) 23 in whichthe volume ratio of the pearlite 7 in the structure is lower by not lessthan 50% than the volume ratio of the pearlite 7 in the structure of thecenter portion of the back-metal layer 2. The low-pearlite region 23extends from the bonding surface 21 and crosses the pore existing region5, and is located also on an inner side in the thickness direction ofthe back-metal layer (see FIG. 5).

A thickness T2 (thickness from the bonding surface 21) of thelow-pearlite region 23 is preferably 1.1 to 1.5 times the thickness T1of the pore existing region 5 (T2/T1=1.1 to 1.5) in cross-sectional viewperpendicular to the sliding surface 31.

The back-metal layer 2 includes a hypoeutectoid steel including 0.07 to0.35 mass % of carbon. The hypoeutectoid steel has a structure includingthe ferrite phase 6 and the pearlite 7. A ratio of the pearlite 7 in thehypoeutectoid steel is determined according to the amount of carbonincluded in the steel, and is typically not more than 30 volume %. Thecenter portion of the back-metal layer 2 has such a typical structure ofa hypoeutectoid steel.

However, the volume ratio of the pearlite 7 in the structure of the poreexisting region 5 on the bonding surface 21 of the back-metal layer 2serving as the interface with the sliding layer 3 is lower by not lessthan 50% than the volume ratio of the pearlite 7 in the structure of thecenter portion of the back-metal layer 2. A difference in elasticdeformation between the ferrite phase 6 and the copper alloy is smallerthan a difference in elastic deformation between the pearlite 7 and thecopper alloy. Accordingly, when an external force is applied to thesliding member, a difference in elastic deformation is small at aninterface between the copper alloy 4 of the sliding layer 3 and the poreexisting region 5 of the back-metal layer 2, and thus shear failure isless likely to occur at the interface, leading to stronger bondingbetween the copper alloy 4 of the sliding layer 3 and the back-metallayer 2.

On the other hand, the back-metal layer 2, except for the low-pearliteregion 23, has a structure of a hypoeutectoid steel including a typicalamount of pearlite, and thus has high strength required for theback-metal layer 2. Therefore, the sliding member 1 is less likely to beplastically deformed by circumferential stress applied when the slidingmember is mounted to the bearing housing or circumferential forceapplied to the sliding member during operation of the bearing device.

A method of producing a sliding member according to the presentembodiment is described below.

First, a copper alloy powder having the composition described above fora sliding layer is prepared. When producing a sliding layer includingthe hard particles and/or the solid lubricant, a mixed powder of thecopper alloy powder and the hard particles and/or the solid lubricantparticles is prepared.

The prepared copper alloy powder or mixed powder is scattered on a steel(hypoeutectoid steel) plate having the composition described above, andthen subjected to first sintering in a reducing atmosphere at atemperature of 800 to 950° C. in a sintering furnace without applyingpressure to the scattered powder to form a porous copper alloy layer onthe steel plate, followed by cooling to a room temperature.

Next, first rolling is performed to make the porous copper alloy layerdense and activate a portion in the vicinity of a surface of the steelplate in contact with the porous copper alloy layer. In conventionalproduction of a sliding member, first rolling has been performed for thepurpose of reducing pores of a porous copper alloy layer to make theporous copper alloy layer dense, and a steel plate has hardly beenrolled. However, in the production of a sliding member according to thepresent invention, a rolling ratio in the first rolling is higher thanin the conventional production, and the rolling is continued even afterthe porous copper alloy layer is densified. Before the first rolling,the porous copper alloy layer has lower hardness than the steel plate.However, until pores of the porous copper alloy layer are reduced tomake the porous copper alloy layer dense by the first rolling, only theporous copper alloy layer is plastically deformed and thus sufficientlywork hardened. Furthermore, when the densified and work hardened porouscopper alloy layer is further rolled, the hardness relationship isreversed and the porous copper alloy layer has higher hardness than thesteel plate (e.g., a surface of the densified copper alloy layer of therolled member after the first rolling has Vickers hardness higher byapproximately 15 Hv than Vickers hardness of a back surface of the steelplate), and the steel plate starts to be rolled. Thus, by the firstrolling, the portion in the vicinity of the surface of the steel platein contact with the copper alloy layer having increased hardness isactivated due to a larger amount of crystal distortion introduced to theportion in the vicinity of the surface of the steel plate than to aninner portion of the steel plate.

Next, the rolled member is subjected to a recovery treatment at atemperature of not lower than 650° C. but lower than a recrystallizationtemperature of the steel back-metal (e.g., less than 700° C.) for aholding time of 2 to 10 minutes in a reducing atmosphere in a sinteringfurnace. Then, the rolled member is subjected to second sintering in areducing atmosphere of 800 to 950° C. to sinter the copper alloy layer,and cooled to a room temperature. At this time, a pore existing regionis formed on a surface of a back-metal layer serving as an interfacewith the copper alloy layer.

A mechanism of forming the pore existing region is presumed as follows.

In the above recovery treatment, a recovery phenomenon occurs in thesteel plate. The recovery phenomenon is such a phenomenon that when asteel plate is heated to a temperature of lower than a recrystallizationtemperature of the steel plate, a part of crystal distortion (atomicvacancy) introduced to an Fe atomic arrangement by the first rolling(cold working) moves (diffuses) to a surface (bonding surface) of thesteel plate and is eliminated.

The part of the crystal distortion (atomic vacancy) introduced to theportion in the vicinity of the surface of the steel plate in contactwith the copper alloy layer densified by the first rolling is moved bythe recovery process toward the surface side of the steel plate incontact with the copper alloy layer. During the recovery process,simultaneously, Ni and P included in the copper alloy diffuse in thevicinity of the bonding surface of the steel plate to be replaced withthe atomic vacancy. Diffused Ni and P serve as resistance to diffusion(movement) so that the crystal distortion (atomic vacancy) which existat a portion inner side of the steel plate than the region where Ni andP diffused is prevented to move toward the bonding surface. Thus,presumably, most of the crystal distortion (atomic vacancy) in theportion at the inner side of the steel plate than the region where Niand P diffuse is partially aggregated to form closed pores in thevicinity of a boundary between the region where Ni and P diffuse and theregion where Ni and P do not diffuse.

However, when the copper alloy includes less than 0.06 mass % of P, Niin the copper alloy is less likely to diffuse in the vicinity of thesurface of the steel plate, and thus a pore existing region is lesslikely to be formed. Although the details are unclear, presumably, whenthe copper alloy includes a large amount of P (not less than 0.06 mass%), a large amount of Cu—P-based liquid phase is generated in the copperalloy in the recovery process, thereby prompting diffusion of Ni and Pin the vicinity of the surface of the steel plate.

In the first rolling, when the surface of the densified copper alloylayer of the rolled member has Vickers hardness higher by not less thanapproximately 20 Hv than the Vickers hardness of the back surface of thesteel plate, the low-pearlite region 23 is formed on the surface of theback-metal layer serving as the interface with the copper alloy layer.

A mechanism of forming the low-pearlite region 23 is presumed asfollows.

Before a temperature reaches an A1 transformation point (727° C.) duringthe temperature rise in the second sintering process, recrystallizationphenomenon occurs, in the back-metal layer (steel plate) of the rolledmember, earlier in the vicinity of the bonding surface serving as theinterface with the copper alloy layer which is activated as comparedwith the inner portion. Thus, immediately before the temperature reachesthe A1 transformation point, a ratio between the ferrite phase and thepearlite are the same in the structure in the vicinity of the bondingsurface of the back-metal layer and in the structure of the innerportion of the back-metal layer, but the ferrite phase has larger grainsin the vicinity of the surface than in the inner portion.

When the temperature reaches the A1 transformation point, the pearliteof the back-metal layer is transformed to an austenitic phase, and theback-metal layer becomes a structure composed of the ferrite phase andthe austenitic phase. (Immediately after the temperature reaches the A1transformation point, a ratio of the austenitic phase and aconcentration of carbon solid-solved in the austenitic phase in thestructure of the back-metal layer do not differ between the portion inthe vicinity of the bonding surface and the inner portion of theback-metal layer.) After that, during the temperature rise until thetemperature exceeds the Al transformation point and reaches an A3transformation point (a temperature at which the structure becomes astructure of the austenitic single phase), the ferrite phase in thestructure is gradually transformed to the austenitic phase in theback-metal layer, leading to reduction in the ratio of the ferrite phasein the structure.

The ferrite phase in the structure in the vicinity of the bondingsurface of the back-metal layer has a larger grain size and is morestable than the ferrite phase in the structure of the inner portion ofthe back-metal layer, and is thus less likely to be transformed to anaustenitic phase. Throughout the temperature rise, a ratio of theaustenitic phase in the structure in the vicinity of the surface of theback-metal layer is lower than a ratio of the austenitic phase in thestructure of the inner portion of the back-metal layer.

Carbon atoms are hardly solid-solved (approximately 0.02 mass % atmaximum) in the ferrite phase in the structure, and thus carbon atomshaving included in the pearlite are solid-solved in the austenitic phasein the vicinity of the surface of the back-metal layer. However, theamount of carbon atoms solid-solved in the austenitic phase in thevicinity of the surface of the back-metal layer is smaller (in volume)than the amount of carbon atoms solid-solved in the austenitic phase inthe inner portion of the back-metal layer. Thus, a concentration ofcarbon differs between the austenitic phase in the vicinity of thesurface of the back-metal layer and the austenitic phase in the innerportion of the back-metal layer. In order to eliminate the difference inthe concentration, the carbon atoms having included in the austeniticphase in the vicinity of the surface diffuse in the austenitic phase inthe inner portion, and thus the amount of carbon included in thestructure in the vicinity of the surface becomes smaller than the amountof carbon included in the structure of the inner portion.

In the subsequent cooling process, when the temperature reaches the Altransformation point, the back-metal layer becomes a structure composedof the ferrite phase and the pearlite. However, after the cooling, thevolume ratio of the pearlite in the structure in the vicinity of thesurface presumably becomes lower than the volume ratio of the pearlitein the structure of the inner portion during the temperature risedescribed above for the following reasons:

(i) The amount of carbon included in the structure in the vicinity ofthe bonding surface serving as the interface with the copper alloy layeris smaller than the amount of carbon included in the structure of theinner portion; and(ii) The volume ratio of the ferrite phase remaining in the structurediffers between the structure in the vicinity of the surface and thestructure of the inner portion during the temperature rise.

In the conventional production of a sliding member, first rolling isperformed only such an extent that a porous copper alloy layer isdensified, and thus a back-metal layer is not rolled. This rolling doesnot activate a portion in the vicinity of an interface between theback-metal layer (steel plate) and the densified copper alloy layer tointroduce a larger amount of crystal distortion in the vicinity of theinterface than in an inner portion of the back-metal layer. Therefore,even when a recovery process is performed in the same conditionsdescribed above, no pore existing region is formed in the structure ofthe back-metal layer after the subsequent second sintering process, andthe volume ratio of pearlite does not differ between the structure inthe vicinity of the surface and the structure of the inner portion.

Even when a member including a copper alloy layer and a back-metal layersubjected to rolling for densification and then second sintering (as inJP 2006-22896A) is further subjected to second rolling to roll both thecopper alloy layer and the back-metal layer, since the copper alloyalready has lower hardness than the back-metal layer by heat treatmentduring the second sintering and the copper alloy layer is alreadydensified, the second rolling does not cause only the copper alloy layerto be plastically deformed (work hardened). Thus, the second rollingdoes not cause the copper alloy layer to be more work hardened to havesufficient hardness than the back metal, thereby preventing activationof only a portion in the vicinity of an interface with the copper alloylayer to introduce a larger amount of crystal distortion to the portionin the vicinity of the interface than to the inner portion of theback-metal layer. Therefore, even when the rolled member is subjected toa recovery process in the same conditions as described above and thirdsintering in the same conditions as the sintering conditions of thesecond sintering, no pore existing region is formed, and the ratio ofpearlite does not differ between the structure in the vicinity of thesurface and the inner portion of the back-metal layer.

The sliding member of the present invention is not limited toapplication to a bearing used in an internal combustion engine or anautomatic transmission, and can be applicable to a bearing used invarious machines. Furthermore, the shape of the bearing is not limitedto a cylindrical shape or a semi-cylindrical shape. The sliding memberof the present invention is applicable, for example, to an annular orsemi-annular thrust bearing that bears an axial load of a shaft member,an annular end plate having a substantially U-shaped cross section usedin a clutch (one-way clutch) of an automatic transmission, and the like.

1. A sliding member comprising: a back-metal layer having a back surfaceand a bonding surface; and a sliding layer on the bonding surface of theback-metal layer, the sliding layer being made of a copper alloy andhaving a sliding surface, the copper alloy comprising 0.5 to 12 mass %of Sn, 0.5 to 15 mass % of Ni, 0.06 to 0.2 mass % of P, and the balanceof Cu and inevitable impurities, wherein the back-metal layer is made ofa hypoeutectoid steel comprising 0.07 to 0.35 mass % of carbon, and hasa structure comprising a ferrite phase and pearlite, wherein theback-metal layer includes a pore existing region, the pore extendingregion including a plurality of closed pores that are not open to thebonding surface when viewing a cross-section perpendicular to thesliding surface, the closed pores having an average size of 5 to 15 μm,the pore existing region extending from the bonding surface toward aninner portion of the back-metal layer and having a thickness of 10 to 60μm, wherein a ratio V2/V1 of a total volume V2 of the closed pores to avolume V1 of the pore existing region is 0.05 to 0.1.
 2. The slidingmember according to claim 1, wherein an average distance between theclosed pores and the bonding surface is 5 to 15 μm in thecross-sectional view perpendicular to the sliding surface.
 3. Thesliding member according to claim 1, wherein an average distance betweenadjacent ones of the closed pores in a direction parallel to the bondingsurface is 5 to 15 μm in the cross-sectional view perpendicular to thesliding surface.
 4. The sliding member according to claim 1, wherein anaverage aspect ratio of the closed pores is not more than 2.5 in thecross-sectional view perpendicular to the sliding surface.
 5. Thesliding member according to claim 1, wherein a volume ratio Pc ofpearlite in a structure of a center portion in a thickness direction ofthe back-metal layer and a volume ratio Ps of pearlite in the poreexisting region satisfy the following relation:Ps/Pc≤0.5.
 6. The sliding member according to claim 1, wherein a regionof the back-metal layer excluding the pore existing region has acomposition including 0.07 to 0.35 mass % of C, not more than 0.4 mass %of Si, not more than 1 mass % of Mn, not more than 0.04 mass % of P, notmore than 0.05 mass % of S, and the balance of Fe and inevitableimpurities.
 7. The sliding member according to claim 1, wherein thecopper alloy further includes one or more selected from 0.01 to 10 mass% of Fe, 0.01 to 5 mass % of Al, 0.01 to 5 mass % of Si, 0.1 to 5 mass %of Mn, 0.1 to 30 mass % of Zn, 0.1 to 5 mass % of Sb, 0.1 to 5 mass % ofIn, 0.1 to 5 mass % of Ag, 0.5 to 25 mass % of Pb, and 0.5 to 20 mass %of Bi.